This invention relates to phased array antenna systems, and, more particularly, relates to optical true time delay generation methods and architecture for such systems.
The phased array antenna is one of the most advanced radar technologies which allows multiple beam pointing and fast non-mechanical steering of microwave beams. The technology has promise for broad-band (2-20 GHz) free-space radar communications that can be used for a variety of commercial and military applications. Beam pointing/steering control systems are known, including true time delay systems and phase shift systems, for phased array antenna, true time delay systems being preferable since the steered beam angle is independent of frequency and squint is eliminated.
For a given microwave frequency  and number of microwave radiating elements along one direction N, the maximum time delay xcex94tmax required to steer a beam over xc2x190xc2x0 is given by N/f. Also, the minimum temporal resolution xcex94xcfx84min to achieve resolution R is given by 1/(fxc2x7R). Assuming a frequency range of 2-20 GHz, N=100 and R=1,000, xcex94tmax=5-50 nsec and xcex94xcfx84min=0.05-0.5 psec.
In conventional electronic RF systems, true time delay is achieved using switched lengths of electrical waveguide or cable. Such devices tend to be bulky, expensive, have high loss at high frequencies, and are susceptible to electrical crosstalk (due to electromagnetic interference) and temperature induced time delay changes. Recent advances in photonic technology can provide a better implementation of true time delay due to a natural high parallelism and large bandwidth as well as immunity to electromagnetic interference.
Heretofore known or suggested photonic true time delay systems have been configured so that each microwave element requires R fixed time delay generators, R switches and an R to 1 combiner. Thus, for a two dimensional (2-D) array with N2 elements in such systems, N2R time delays and N2R switches have been required. The insertion loss is mainly determined by the R to 1 combiner and is given by 10 log10R. Although such a system is capable of adaptive beam forming as well as beam steering, it requires a tremendous amount of complexity, making its hardware implementation extremely difficult.
Although this complexity can be reduced to some degree by free-space path-switching methods, this still requires a cascaded array of many independent time-delay generators and parallel (N2) switches in 2-D spatial light modulators. Moreover, thus configured, the system presents other limitations, such as speed and path-dependent insertion loss.
A highly dispersive fiber prism method has been suggested and/or utilized that can significantly reduce the complexity as described above. However, this method requires very long (20 km for 1 GHz), N2 fiber bundles and a fast tunable narrow linewidth light source with broad tuning range. It has been suggested that the long length could be significantly reduced by using an array of fiber gratings, but significant problems with this implementation would yet be posed. Most of the heretofore suggested approaches for use of fiber gratings as a means to generate true time delays employ an array of normal single frequency fiber gratings, the desired time delays being selected by a tunable narrow linewidth light source. To achieve high resolution, both a broad tuning range and a narrow linewidth are required. Moreover, the wavelength would need to be changeable rapidly (within a few microsecondsxe2x80x94a speed unattainable by current laser technology) for effective implementation.
In addition, two dimensional (2-D) extension architecture for such photonic true time delay systems as have been heretofore suggested could utilize further improvements. Conventional image rotation has been accomplished, for example, by rotating a dove prism by an angle 0 around the optical axis, the output image thus being rotated by 20. Such conventional rotation thus requires mechanical movement of components and is, therefore, inherently slow and lacking adequate unreliability.
This invention provides a true time generating system and method for both one dimensional and two dimensional generation of time delayed gratings for use with phased array antenna systems. This invention includes a delay encoder operable with a broadband light source and utilizing a single optical fiber having an array of fiber chirp gratings therein, an optical signal decoding device for receiving a wavelength encoded light signal and providing as an output therefrom a time delayed grating, and image rotation utilizing acousto-optics and without moving mechanical parts.
The delay encoder fiber chirp gratings are configured so that different wavelengths of the light from the light source are reflected at unique locations at each individual fiber chirp grating, the locations corresponding to different selected time delays, thereby providing a wavelength encoded light signal output. The decoding device receives the wavelength encoded light signal output and utilizes this output to provide a time delayed linear grating as an output therefrom.
An optical amplifier amplifies the wavelength encoded light signal and an acousto-optic deflector having a variable acoustic signal input is positioned to receive the amplified light and disperse the light at selected diffraction angles variable by an acoustic signal at the input. A window is positioned at the output plane from the deflector for selection of an output spectrum from the dispersed light, spectrum selection controlled by diffraction angle selection at the acousto-optic deflector.
The method for generating true time delays of this invention includes the steps of launching broadband light into an optical fiber having a plurality of selectively located fiber chirp gratings therealong to provide a wavelength encoded light signal output. The light signal output is dispersed at selected diffraction angles to provide multiple wavelength spectra linearly arrayed at an output plane, a spectrum from the multiple wavelength spectra at the output plane corresponding to a selected one of the fiber chirp gratings at said optical fiber being selected thereby providing a selected time delayed linear grating.
Utilizing this invention, the complexity of heretofore known systems can be significantly reduced, requiring only a number of fiber chirp gratings in a single fiber providing the number of different time delays desired. The compact system allows a broad range of time delays that can be reconfigured within a few microseconds, and because of fewer or no mechanical elements and switches, is inherently more reliable.
It is therefore an object of this invention to provide true time delay generation systems and method utilizing a broadband light source and a fiber chirp grating array in a single fiber.
It is another object of this invention to provide true time delay generation systems and method including fully optical delay selection and acousto-optic image rotation.
It is another object of this invention to provide optical true time delay generation systems of reduced complexity, requiring only a number of fiber chirp gratings in a single fiber providing the number of different time delays desired.
It is still another object of this invention to provide true time delay generation systems for phased array antenna systems which are compact, allow a broad range of time delays that can be reconfigured within a few microseconds, and are highly reliable.
It is still another object of this invention to provide a true time delay generating system including a broadband light source, delay encoding means including an optical fiber for receiving light from the broadband light source, the optical fiber having at least a first selectively located fiber chirp grating defined therein so that different wavelengths of the light from the light source are reflected at unique locations at the fiber chirp grating, the locations corresponding to different selected time delays, thereby providing a wavelength encoded light signal output, and decoding means for receiving the wavelength encoded light signal output and utilizing the wavelength encoded light signal output to provide a time delayed linear grating as an output therefrom.
It is yet another object of this invention to provide a true time delay generating system including a broadband light source, delay encoding means including an optical fiber for receiving light from the broadband light source, the optical fiber having at least a first selectively located fiber chirp grating defined therein so that different wavelengths of the light from the light source are reflected at unique locations at the fiber chirp grating, the locations corresponding to different selected time delays, thereby providing a wavelength encoded light signal output, and decoding means for receiving the wavelength encoded light signal output and utilizing the wavelength encoded light signal output to provide a time delayed linear grating as an output therefrom, the decoding means including an acousto-optic deflector for light dispersion at diffraction angles selectable by variation of an input signal.
It is still another object of this invention to provide an optical signal decoding device for receiving a wavelength encoded light signal and providing as an output therefrom a time delayed grating for utilization in a phased array antenna system, the device including an acousto-optic deflector for receiving an amplified light signal and dispersing the light signal at selected diffraction angles to an output plane, and a window at the output plane for selection of an output spectrum from the dispersed light signal.
It is yet another object of this invention to provide a method for generating true time delays including the steps of launching broadband light into an optical fiber having a plurality of selectively located fiber chirp gratings therealong to provide a wavelength encoded light signal output, dispersing the wavelength encoded light signal output at selected diffraction angles to provide multiple wavelength spectra linearly arrayed at an output plane, and selecting a spectrum from the multiple wavelength spectra at the output plane corresponding to a selected one of the fiber chirp gratings at the optical fiber to thereby provide a selected time delayed linear grating.
With these and other objects in view, which will become apparent to one skilled in the art as the description proceeds, this invention resides in the novel construction, combination, arrangement of parts and method substantially as hereinafter described, and more particularly defined by the appended claims, it being understood that changes in the precise embodiment of the herein disclosed invention are meant to be included as come within the scope of the claims.